This invention relates to contact means for semiconductor devices and, in particular, to edge protected bonding pedestals for attachment to a hard metal leadframe.
Multileaded semiconductor devices such as monolithic integrated circuits have traditionally been provided with external connections by wire bonding between the small bonding pads integral with the intra-device metallization and external metallic conductors which are mechanically supported by the package. To decrease costs and increase reliability, various structures for gang-bonding to external metallic connections have been utilized or proposed. For example, solder pedestals have been provided over the bonding pad locations by evaporating solder through a mask. The individual circuits are then soldered to an external metallization pattern such as a printed circuit. It is also known to selectively plate the bonding pads in order to provide pedestals suitable for gang-bonding of a soft metallic leadframe which is subsequently bonded to a hard external leadframe.
In any of these processes, it is common to provide an insulating layer over the intra-circuit metallization in order to protect (passivate) the internal connections during assembly of the semiconductor element. If the bonding pads are formed solely by the metallization used for the intra-circuit connections, the passivating layer is typically formed over the metallization and bonding pad structures and then etched to provide access to the bonding pads.
If bumps are to be found, additional film or films are deposited in the apertures, the height of the bump structure being increased by subsequent plating operations. Known metallurgy of such bump structures comprise an alternating sequence of at least two different metals in which the potential for electrochemical corrosion is inherent. In the more conventional bumpless structure, the fact that the passivating layer is the highest point at the bonding pad site makes it highly vulnerable to damage during the bonding operation, because the space allocated to the bonding pad may be sufficiently small that the possibility of application of the high bonding pressure to the passivating layer is significant. If the passivating insulating layer is cracked or damaged as a result of this operation, crevice regions at the edge of the metallization are exposed to the package ambient, which often contains corrosion-inducing materials. The possibility of metallization corrosion failure is highest at the crevice exposures of the metal edges because of the higher electrochemical potential electric fields thereat. Thus either monometal or bimetal systems are prone to corrosion when their lateral edges are not sealed.
In view of the foregoing, it is an object of this invention to provide a semiconductor bonding pedestal having a raised portion to allow lead bonding without damage to the insulator which seals the semiconductor device metallization pattern except at the broad faces of the bonding pad.
It is a further object of this invention to provide such a raised edge, passivated pedestal without adding significantly to the complexity of the processes utilizing the fabrication of the semiconductor element.
It is yet a further object of this invention to provide elevated edge-passivated pedestals which are pedestals which are suitable for direct gang-bonding of a planar self-supported metallic leadframe substantially harder than both known copper, nickel or aluminum leadframe materials and the metallization used for the intra-device interconnections.
The improved bonding pedestal of the present disclosure comprises a non-planar metal bonding layer. The non-planar metal bonding layer is lowest at the periphery of the bonding pad and is provided with a support element approximately central to the bonding pad. This support element may be formed of the same metal as the bonding pad, may be an insulator formed as a result of the semiconductor structure fabrication, may be a metal layer of different composition than the bonding pad, or some combination of the foregoing. The central support structure has a height somewhat greater than that of the passivation used to cover the intra-device metallization. This passivation then covers only the peripheral lower portion of the metal bonding layer which then becomes the highest point in the bonding pedestal structure where it overlies the support element. In such a structure the lateral edges of the metallic bonding pad are completely covered and sealed by a passivating layer which is not subject to damage by the bonding process. The metal bonding layer, in turn, passivates the metal layers of the support element.